It has already been known to dye cotton warp yarn in a continuous process named chain dyeing; see, e.g., Fischer-Bobsien, Internationales Lexikon Textilveredlung, 4th ed., 1975, p. 286; and P. Richter in "Textilveredlung" 10, 1975, p. 313-7. In this process, yarn cables called ball warps, consisting each of about 350 to 400 single warp yarns, are introduced in parallel relationship into a dipping vat containing the reduced leuco form of indigo. Then the excess of vat is squeezed from the yarn cables or bundles, and the vat dye thereon is allowed to oxidize in air to give the blue indigo. The yarn cables or ball warps are then repeatedly immersed in the vat, squeezed and exposed to air until the desired color depth is obtained. Finally, the ball warps are rinsed one or two times, the rinsing liquor is squeezed out, and the ball warps are dried on a drying cylinder after an optional livening treatment. The ball warps coming from the drying cylinder must now be opened or rebeamed, and they are then wound up on several loom beams.
This method has serious drawbacks since it is necessary to install a special dyeing machine equipped with special units for the forming of the ball warps and the rebeaming of the dyed yarn cables. In particular, the rebeaming step is rather difficult and ticklish and requires very skilled workmen. A further drawback is the fact that the obtained dyeing is always unlevel since there are irregularities when the yarn cables are immersed into the vat, during squeezing off the vat excess, and during air oxidation. Other inequalities result from the fact that adjacent ball warps are often not in touch with each other.
These unlevel results may partially and statistically be compensated by mixing the yarns during the rebeaming step and during the winding up on the loom beams. On the fabric made from such a dyed warp yarn, clearer and darker spots are alternating so that the aspect of the fabric is the characteristic one of the known "real indigo" fabrics.
Efforts have already been made to improve the dyeing level in continuous dyeing methods of cotton warp yarn with indigo vat, and to avoid the complexity of the chain dyeing machine. It has been tried to adapt the existing sizing or slashing machines to the vat indigo dyeing process.
In the well-known sizing or slashing machines, the yarn, being unwound from front warp beams, is continuously immersed as a row of parallel yarns, in the width of the future fabric, into a trough containing a sizing bath. The yarns are impregnated with the sizing liquor, and the excess thereof carried away by the yarn is removed therefrom between squeeze rollers. The squeezed yarns are dried, normally on drying cylinders, and finally wound into a final warp beam or loom beam.
Attempts have already been made to effect continuous vat dyeing of warp yarns on a modified slasher; see the already cited publication in "Textilveredlung" and M. Peter, Grundlagen der Textilveredlung, Stuttgart 1970, p. 178. This method is sometimes called "slasher dyeing" since it is basically performed like the sizing or slashing mentioned above. The method comprises passing the warp yarns, not in the form of bundles or cables made from a plurality of warp yarns, but in the form of a substantially parallel thread row, through the indigo vat, squeezing an excess of vat from the yarns, and exposing the squeezed yarns to air for oxidation of the leuco indigo, repeating the steps of vat immersion, squeezing and oxidizing until the desired color density is reached (at least three times, generally four to six times), washing the yarn, optionally aviving it, and winding the yarn on a loom beam.
In this method too, it is always found that the resulting dyeing is not level. This unlevelness consists in the fact that some yarns are darker and some are lighter dyed than the bulk. Whereas the overall unlevelness of the chain dyeing method may be equalized in mixing the yarns from the ball warps, the unlevelness of the slasher dyeing cannot be compensated in this way, since after weaving the dyed warp yarns, the fabric shows darker and lighter strips. It has not been possible until now to eliminate these dyeing defects.
In the method of slasher dyeing, the linear thread density in the squeezing zone is not higher than 1.25 and generally lies in the range from 0.9 to 1.2. The linear thread density Q is defined as the product of the thread count F (in cm.sup.-1) and the yarn diameter D (in cm): EQU Q = F .times. D.
of course, F and D may be expressed in other length units, like inch, as far as they are the same for F and D. The thread count is a current expression in the field of weaving; it defines the number of substantially parallel threads or yarns over a given width of fabric or similar thread arrangements. In the conventional techniques of slasher dyeing, the thread count in the nip between the squeezing rollers substantially equals the thread count on the loom beam.
It has been supposed that the accumulation of yarns in certain regions of said squeeze nip, i.e. the overlapping of two or more yarns in these regions, would be responsible for the unlevel dyeing. In these regions, the squeezing force is particularly high, and the amount of vat remaining on the yarns in said regions is particularly low. However, experiments wherein the thread count in said squeeze nip and, consequently, the linear thread density, were reduced, have not proved successful.
The working techniques of a slasher dyeing machine fully correspond to those of a warp sizing machine. The warp yarns sized in the latter present exactly the same unlevelness of sizing as that described above for the indigo dyeing on such a machine, modified for vat dyeing, with the exception that unlevel sizing is not visible. It has not been possible until now to explain the fact that there arise sometimes problems during weaving, and these problems were attributed to a possibly too high squeezing ratio in the sizing equipment. Consequently, for sake of safety during weaving, the warp yarns have always been oversized which is a waste of material, time and energy.
With the foregoing in mind it is here to be noted that the present invention has for one of its primary objectives to improve the techniques in the slasher dyeing of cotton warp yarn with indigo vat dye, and to improve the technique of warp yarn sizing.
A further and more general objective is to provide a new process wherein the principle of the known slasher dyeing and sizing machines is applied to all possible finishing operations normally carried out on warp yarns of any kind, in such a manner that, in one aspect, finishing operations become possible which could not yet be performed on such machines, and, in another aspect, such treatments impart perfectly level finishing effects on warp yarns.
A further and more specific objective is to provide an improved method selected from the group consisting of dyeing of warp yarns which enjoy the notable advantage of giving very level effects, namely a level dyeing and a level sizing.
There is another objective of the instant invention to permit, at the same time, a substantially improved production output.
Now in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the invention is directed to continuously treating warp yarns by a method, wherein a row of substantially parallel, unbundled warp yarns is impregnated with an excess of a treating liquor, said excess is then removed from the impregnated yarns in a liquor removing zone, the yarns are dried and finally wound up on a loom beam, and contemplates that, at least in said liquor removal zone, the linear thread density of the warp yarn row, expressed by the relation Q = F .times. D, wherein Q is the linear thread density, F is the thread count in cm.sup.-1, and D is the mean diameter of each thread in cm, is brought to a value of at least 2, said value of at least 2 being constant over the entire width of said yarn row and being the same at any width increment, and the finally dried warp yarns are wound up on at least one loom beam, said increased linear thread density of at least 2 being reduced to the nominal thread count for weaving of each loom beam before the treated warp yarns are wound up on said beam.